Described in my U.S. Pat. Nos. 4,206,761, 4,206,762, 4,281,666, 4,281,667, and 4,385,636 was a method of measuring intracranial pressure (ICP) using a known pressure applied to the scalp over the implanted sensor, so as to calibrate the sensor in-vivo, and eliminating the need to know the sensor calibration before implantation. The applied pressure was arbitrary, and no restriction on its value was suggested. In particular, positive or negative applied pressure are equally suitable, as the basic premise of the invention is that the scalp over the sensor transmits pressure faithfully and accurately to the sensor. It was also described in my U.S. Pat. Nos. 4,206,761, 4,206,762, 4,281,666, 4,281,667, and 4,385,636, and in the papers by Zervas, Cosman, and Cosman (1977) and Cosman, et al (1980) that a free-running, continuous readout of the sensor diaphragm displacement, or equivalently some parameter which changes with that displacement, would be a measure of ICP if one calibrated the sensor's response curve by applying the scalp pressure at various values. In the papers above, such continuous graphical recordings were shown for which the pressure axis was calibrated in-vivo in that way.
The need for measuring negative ICP has, in recent years, become more important. This continuation-in-part describes a specific variant of our general method which applies to negative ICP monitoring and for calibration in-vivo of a free-running readout of the sensor diaphragm movement. Thus it is a continuation of previous method and apparatus.